Free machining austenitic stainless steel



g- 1969 .1. A. FERREEQJR FREE MACHINING AUSTENITIC STAINLESS STEEL 2 Sheets-Sheet 1 Filed May 29, 1968 Q .l G F E s E S E M E S A N F. G A N N G A A N m M M A 3 m 8 o 6 4 2 +l u n u u u O A 2 9 B 2 2 2 2 2 I. I .1 l

MEI QED OOJ PERCENT COPPER FlGJb I '2 PERCENT COPPER INVENTOR. .JOS'EPH A.FERREE,JR. BY M w W ATTORNEY United States Patent US. Cl. 75125 6 Claims ABSTRACT OF THE DISCLOSURE Described herein is a method of making a free machining austenitic stainless steel and the product produced thereby. The method comprises adding alloying additions to a steel melt to produce a steel consisting essentially of from a trace up to .15% carbon, from 2% to 10% manganese, from 4% to 13% nickel, from 10% to 20% chromium, from .5 to 3% copper, from .10% to .40% sulfur, 2% max. silicon, and .10% max. nitrogen, the balance essentially iron and residual impurities, the improvement which comprises adding alloying additions, as described, to a steel melt and controlling the constituents so that the delta ferrite-forming characteristic is less than 10 according to the formula:

delta ferrite potential=percent Cr+l.5 (percent Si) .87[3O (percent C+percent N)+percent Ni+0.5

(percent Mn) +0.3 (percent Cu)] +1 and the amount of copper does not exceed 3.850.18 (percent Mn S).

This application is a continuation-in-part of application Ser. No. 418,991, filed Dec. 17, 1964 now abandoned.

This invention relates to ferrous base alloys, and more particularly to a free machining, austenitic stainless steel.

Of the many different grades of stainless steel, AISI Type 303 is the conventional, standard type free machining austenitic stainless steel. This type of stainless steel, which is a chromium-nickel type, contains, as principal alloying components, from about 17% to 19% chromium, from 8% to 10% nickel, .15 maximum carbon, 2% maximum manganese and 1% maximum silicon, with up to about 20% phosphorus; up to .60% molybdenum and .60% zirconium may be added for some applications. It has been found, however, that an austenitic stainless steel with substantially improved free machining properties over those of AISI Type 303 is provided in chromiumnickel-manganese-copper type austenitic stainless steel to which sulfur is added as a free machining element. Additionally, it has been found that the same chromiummanganese-copper system will provide a stainless steel of equivalent machinability to that of the Type 303 stainless steel with about half of the amount of sulfur that would be required in the Type 303 stainless steel. Since the sulfur content is lower the corrosion resistance is improved, as are the hot and cold working properties, inasmuch as the detrimental eifects of sulfur on corrosion resistance and hot and cold working are well known.

It is therefore a principal object of this invention to provide an improved free machining austenitic stainless steel.

A more particular object of this invention is to provide an austenitic, free machining stainless steel which will have machinability comparable to that of the AISI Type 303 at reduced sulfur levels, and improved machinability at comparable sulfur levels.

A still further, more specific object is to provide an improved free machining austenitic stainless steel where- ICC in the alloying elements are balanced to produce optimum corrosion resistance and free machining properties.

These and other objects, together with a fuller understanding of the invention, may be had from reference to the following description taken in conjunction with the accompanying drawing in which:

FIGURE 1(a) is a graph showing the effect of copper at various manganese levels on the machinability of stainless steels of this invention;

FIG. 1(b) is a composite of the lines of FIG. 1(a) showing this effect of copper at the average manganese content;

FIG. 2(a) is a graph showing the eifect of manganese at various copper levels on the machinability of stainless steel of this invention;

FIG. 2(b) is a composite of the lines of FIG. 2(a) showing this eifect of manganese at the average copper content;

FIG. 3(a) is a graph showing the effect of nickel at various copper levels on the machinability of stainless steels of this invention, and

FIG. 3(b) is a composite of the lines of FIG. 3(a) showing the eifect of nickel at the average copper content.

Subject matter The present invention relates to an austenitic stainless steel characterized by improved machinability. In particular, the austenitic stainless steel described in the appealed claims displays equivalent machinability to that exhibited by the well-known AISI Type 303 stainless steel at a considerably lower sulfur level than that necessary and usually found in the AISI Type 303 stainless steel. As a result thereof, the steel of the claimed invention has a much improved corrosion resistance over that demonstrated by the Type 303 stainless steel. In addition thereto, where corrosion resistance is not of paramount importance, the same sulfur level will produce free machining properties that are approximately four times greater than those exhibited by the AISI Type 303 stainless with the same sulfur level. These desirable objectives are obtained by critically controlling the alloying components of which the steel is composed, and thereby require a delicate balance in order to obtain the maximum hot workability by controlling the delta ferrite potential. In addition thereto, while the copper is set forth within a given numerical limitation, nonetheless Within this range the maximum copper must be controlled in accordance with the quantity of manganese and sulfur which is present. Accordingly, the crux of the invention lies not only within the novel proportions of the alloying components employed, but also in the delicate balance thereof within the stated ranges to produce an alloy composition having the desired properties.

The free machining characteristics of any ferrous base alloy, and in particular austenitic stainless steel to which this invention is directed, is dependent in large measure upon the percent of free machining elements in the alloy. Although other elements may be used, the most common addition for free machining is sulfur. In austenitic stainless steels, conventionally the minimum amount of sulfur which is added in commercial practice for free machining properties is 0.15%, but most uses require around .3% minimum sulfur to obtain maximum free machining characteristics. I have found that by properly controlling the alloying elements of chromium, nickel, manganese, copper and sulfur, an austenitic stainless steel can be provided which will have materially improved machining characteristics over AISI Type 303 stainless steel at comparable sulfur levels, comparable machining characteristics at reduced sulfur levels, al-

though the machining characteristics will not be as good as the free machining grades of ferritic stainless steels.

The alloy according to this invention will have from a trace up to about .15% maximum carbon, from about 2% to about 10% manganese, from about 4% to about 13% nickel, from 10% to 20% chromium, from .5 to about 3% copper, from .10 to about .40% sulfur, up to about 0.10% nitrogen, and optionally up to about .60% molybdenum and .60% zirconium; other elements may be added to obtain specific characteristics of stainless steel.

Table I below lists the composition and machinability, as measured by drill testing, of various types of free machining stainless steels, some within the cope of this invention and some not. The drill tests are performed in the following manner:

Slabs of material to be tested are provided which are %1" thick and have opposed fiat machined faces. The slabs are chucked in a conventional drill press, and a series of holes is drilled in each slab with twist drills. The twist drills for such testing were manufactured by the Cleveland Twist Drill Company of Cleveland, Ohio, and are similar to conventional twist drills but are finished to the closest possible tolerances. In the present case the drills were 4" diameter drills manufactured from AISI Grade M-1 high speed tool steel. The drill speed for the present tests was 3,050 r.p.m., and the feed was 0.005" per revolution. The feed was automatically accomplished by a screw drive incorporated in the drill press to maintain accuracy. Conventional sulfurized cutting oil was used as a lubricant and maintained at a constant flow throughout the tests. Holes were drilled at least /s" apart to minimize the effect of work hardening radiating from the holes as they were drilled. The tests on the slabs of each heat continued until a wear land of 0.015", as measured by a calibrated microscope, was worn on the cutting edge of the drill used for the test, and this was considered the end point of the test. The total inches drilled was then calculated by multiplying the number of holes drilled by /1 (the thickness of the slabs), and this inches of holes drilled is the value listed in Table I.

TABLE I I II III IV V VI VII VIII Percent Log Log Drill Drill Drill Life Stand- Heat No Cu Mn N1 S Life 1 Life 2 ardized 3 1850* 4 Res. 7. 90 6. 24 28 132 2. 121 2. 207 4 Res 7. 97 8.09 28 109 2.037 2.123 4 Res 8. 7. 07 35 44 1. 644 1. 429 4 Res 8. 20 9. 20 31 74 1. 869 1. 826 4 Res 10. 08 5. 08 27 41 l. 613 1. 742 4 Res 10. 02 6. 87 25 30 1. 477 1. 692 98 2. 03 8. 99 33 43 1. 634 1. 505 1. 09 4. 04 6. 25 35 161 2. 207 1. 992 1. 14 4. 12 7. 83 35 78 1. 892 1. 677 1.04 5. 73 6. 93 30 47 1. 672 1. 672 1. 17 6.07 6. 30 31 95 1. 978 1. 935 1. 23 6. 00 8. 32 172 2. 236 2. 150 1. 7. 94 6. 42 29 163 2. 212 2. 255 1. 07 8. 04 7. 84 31 183 2. 263 2. 220 1. 50 2. 20 6. 65 32 48 1. 681 1. 595 1. 45 2. l3 6. 85 33 101 2.004 1. 875 1. 55 5. 4. 80 41 1. 613 1. 613 1. 56 5. 47 5. 02 33 112 2. 049 1. 920 1. 77 4.09 7. 85 135 2. 130 1. 915 1. 75 5. 5. 30 178 2. 250 2. 250 1. 75 5. 95 5. 15 30 97 1. 987 1. 987 1. 76 5. 90 5. 32 25 75 1. 875 2. 090 1. 75 6. 10 5. 40 32 164 2. 215 2. 129 1. 6. 40 5. 65 32 231 2. 364 2. 278 1. 73 7. 97 8.00 29 143 2.155 2. 198 1. 90 49 7. 00 33 41 1. 613 1. 484 1. 93 1. 05 7. 02 33 119 2. 076 1. 947 2.06 1.12 7. 15 34 184 2. 265 2. 093 1. 99 1. 09 9.14 34 106 2.025 1. 853 1. 92 1. 01 11.15 30 59 1. 771 1. 771 1. 95 1. 94 7. 32 33 82 1. 914 1. 785 1. 98 2.07 6. 66 37 132 2.121 1. 820 1. 98 2. 20 6. 73 34 145 2. 161 1. 989 1. 95 2. 20 6. 38 1. 778 1.434 1. 98 2. 20 7. 15 34 186 2. 270 2.098 1. 98 2. 20 7. 20 35 90 1. 954 1. 739 1. 88 2. 13 9.10 31 1. 903 1. 860 1. 94 4. 00 6. 25 32 129 2. 111 2. 025 1. 88 3. 96 11. 00 33 113 2. 053 1. 924 2.05 6. 00 4. 82 30 111 2.045 2.045 2. 00 6. 38 6. 00 31 133 2. 124 2. 081 1. 6. 08 6. 34 31 171 2. 233 2. 190 1. 6. 00 8. 12 33 216 2. 335 2. 206 1. 96 8. 30 9. 25 29 113 2.053 2. 696 2. 54 4. 08 6. 05 33 150 2. 176 2. 047 2. 49 4.19 7. 90 35 236 2. 373 2.158 2. 65 6. 08 6. 32 31 154 2. 188 2. 145 2. 54 6. 00 7. 93 32 230 2. 362 2. 276 2. 60 8. 05 6. 00 30 234 2. 369 2. 369 2. 46 7. 99 8. 03 29 242 2. 384 2. 427

1 Inches of holes drilled, average of three tests. 2 Logarithm of values in column VI. 3 Values in column VII standardized to .30 sulfur level by the formula:

TABLE I Loggrliltlhgn of Drill Liie-4.30 (percent S-.30)=star1dardized logarithm of i e. 1 H III IV V VI VII VH1 45 4 Residual, 1.9. about 1%.

Percent Log Log Drill Nora-All heats contained from 15 to 19% Or; .15% Max. 0; .10%

Dr1ll Drill Life Stand- Max. N; and normal residual impurities. Heat N0. Cu Mn Ni S Life 1 Life 2 ardized 3 2 1159s {)8 8. 9g .32 59 1. 7;: 771 es. 9. 0 3 53 1. 7 .552 4 Res 1.99 7.07 .33 109 2. 037 1. 908 0 :Res 1.84 9.05 .33 79 1.898 1.769 Of the heats l1sted 1n Table I, the 23 marked with an 1 :22 $138 32 3 j; 513% 1: 3% asterisk were melted as a group having controlled iges. 12 8.18 1 1 0:8 222 analyses, particularly of manganese, copper and nickel, 1 S2: 80 :8 1:774 for the purpcilsle of deltierug rnng itth he effectt;1 of Sale? off thesle 4 Res. 2.26 8.92 35 2. 041 1.826 55 elements on e mac na 1 my. e mac ma 1 1 y 0 care 2 22:: 1:32 .313% g2 35 {$5 of these 23 heats, as well as various average, is tabulated :g s. 352 2 3:2 in Table II below, and these values were used to plot the 4 Res. 5.92 8.02 .30 100 2.000 2. 000 graphs of FIGS- 1 4 Res 5. 92 8.74 .33 5s 1. 763 1. 634 4 Res 6. 06 6.34 31 61 1. 785 1. 742 4 Res. 6.00 6. 92 .32 30 1.477 1.391 60 TABLE 11.-LOG DRILL LIFE [23 Heats Adjusted to 30% Sulfur by the Formula: Log D.L.4.30 (percent S--.30)]

6% Ni 8% Ni Total 4% Mn 6% Mn 8% Mn Total 4% M11 6% Mn 8% Mn Total 4% Mn 6% Mn 8% Mn Total 0% Cu 1.727 1. 742 2. 207 5.676 1. 39s 2. 000 2.123 5. 521 3.125 3. 742 4.330 11.197 A (1846) (1849) (1850) 1 892 (1874) (1848) (1851) 1 840 1 562 1 871 2 165 1 866 1% 03%;: "1:992 "1:955 255' 6: 182 "1:677 "2.150 2251 6:047 31 669 4085 4.475 12.229 A (1852) (1860) (1 2 061 (1856) (1863) (1869) 2 016 1 934 2 042 2 23s 2 8 2% 2.025 2.190 41215 "i.'9i5 "2.206 i913 6319 31940 41 396 2.193 10.534 (1854) (1861) 2 10s (1857) (1864) (1870) 2 106 1 970 2 19s 2 19s 2 107 617""2' ""2: 569' 6: 561 "'2fi5"""'2f276"""2f 6: 861 4: 205 41421 4.796 13.422 1855) (1862) (1868) 2 187 (1858) (1859) (1871) 2 287 2 102 2 210 2 398 2 237 "7f79i"""'f0i2""'"fi' 22: 634 ne eier""r1115? 24.748 14.939 16.644 15.799 47.382 1. 94s 2. 003 2. 277 2. 058 1.787 2.158 2. 242 2.062 1.867 2.080 2. 257 2.060

N out-Heat numbers in parentheses.

It should be noted that the machinability Value used in Table II and in the figures is the logarithm of the drill life standardized to .30% sulfur. The logarithm of the drill life was chosen since this will tend to equalize variations at both high and low values. These logarithms were then corrected to a standardized sulfur content (30% sulfur was chosen since this is commercially the minimum required to obtain maximum machinability in austenitic alloys) to compensate for variation due to different sulfur contents, it being recognized that small variations in sulfur have large efiects on the machinability. The correction factor for sulfur was determined empirically using the heats in Table I. The levels of the elements reported in Table 11 represent the nominal levels of each, although the actual average levels of copper are given in parentheses since they do deviate somewhat from the nominal values. Also, in the figures the various lines are labeled with the nominal values, and in fact the manganese and nickel valuesin FIGS. 2 and 3 are plotted at the nominal amounts since, in the case of these two elements, the nominal values are close to the actual; however, in FIG. 1 the copper values are plotted at the average because of the variation thereof from the nominal.

Referring now to FIG. 1(a), it can be seen that at all manganese levels when the copper is increased from zero to about 2.5%, a substantial increase in drill life occurs, which means the steel is more easily machined at the higher copper levels; also, the machinability is increased at higher manganese levels, as shown by the location of the lines for the various levels of manganese. The composite line shown in FIG. 1(b) shows also the effect of increasing copper as increasing the machinability at the average manganese content; FIG. 2(a) shows that at any given copper level, increasing the percentage of manganese in the steel substantially increases the drill life, thus indicating that at higher manganese levels the ease of machining is substantially increased; also, the location of the lines for the various levels of copper shows increased machinability at higher copper levels; FIG. 2(b) shows also the effect of increasing manganese as increasing the machinability at the average copper percentage; FIG. 3(a) shows that at any given copper level, increasing the nickel content does not have any significant effect on the machinability of the stainless steel, but that the machinability at any nickel level is higher with more copper, and FIG. 3(b) shows this lack of effect of nickel on machinability at average copper levels.

The results of the machinability tests on the various alloys listed in Table I, and particularly the controlled group also listed in Table II and plotted in the figures, indicate that the best machining composition is a composition wherein the manganese and copper are relatively high whereas the nickel content is relatively unimportant with respect to machinability. lt has been found, however, that at this optimum high level of manganese and copper, the composition must be controlled so that the percentage of copper does not exceed 3.85-0.18(percent Mn S); if the copper content exceeds this value, the alloy cannot be properly hot worked because of cracking and edge checking. In fact, the billets from heats 1868 and 1871, the composition of which approaches this limit, gave some indication of edge checking during processing which, if it had been any more severe, would have rendered the material unsuitable. It has further been found, and an examination of Table I will show, that when the manganese content exceeds about 6% there is a tendency for the alloy to retain less than 30% sulfur, which will materially reduce its free machining characteristics since sulfur is the major contributor to the free machining characteristics of the alloy. It is known that more sulfur can be retained in the final product if the melting temperature is raised; however, this introduces additional problems of increased erosion of furnace and ladle refractories which, in turn, may require costly changes in melting, tapping and teeming practices. Hence,

when maximum free machining properties are desired, the sulfur should be in the range of 30% to .40%, which means the manganese content cannot exceed about 6% if conventional furnace and ladle practices are to be followed. However, the machinability of the alloys containing manganese and copper is so superior to that of A151 Type 303 which contains no copper and very little manganese, the sulfur content can actually be lowered to the range of about .15 and machinability comparable to that of AISI 303 can be obtained. For example, heats 1859, 1868 and 1858, which are within the scope of this invention, have a drill life about four times as great as heats 1727 and 1426, which are examples of conventional AISI Type 303 free machining stainless steel. Where increased hot and cold workability, as well as increased corrosion resistance, is desired, and where machinability merely comparable to Type 303 is desired, greater manganese contents can be used with a corresponding reduction in the amount of sulfur, it being understood that the lower the sulfur Value, the greater the corrosion resistance and the greater the workability the alloy will have, but it will have decreased machining characteristics.

It is also known that the alloying elements of stainless steels containing manganeses, nickel and copper must be properly balanced to prevent the formation of excessive delta ferrite during hot rolling. When excessive delta ferrite is formed during hot rolling, the ingot or bloom cannot be properly hot worked; the relationship of the alloying elements to the formation of delta ferrite must be such that the delta ferrite-forming characteristic or potential is less than 10 according to the formula:

delta ferrite potential=percent Cr+1.5 (percent Si).87 [30(percent C-i-percent N) +percent Ni+0.5 (percent Mn) +0.3 (percent Cu) +1 With respect to the composition limits, the chromium content cannot be below about 10% in order to achieve proper corrosion resistance, and the chromium content should not exceed about 20% since more than this would require excessive amounts of other elements to prevent the formation of delta ferrite, and higher amounts of the other elements could lead to hot shortness problems in the balance of copper and manganese and increased expense with respect to nickel. Hence, the broad limits for the chromium are about 10% to 20%. If there is less than about 4% nickel, the manganese and/or copper contents would have to be increased to obtain the stability with respect to the formation of excessive delta ferrite. This would tend to make the alloy hot short if enough manganese and/or copper were added to compensate for the reduced amount of nickel, and also would reduce the amount of sulfur retained if the manganese is increased. More than about 13% nickel adds needlessly and substantially to the cost of the alloy. There must be at least 2% manganese, since less than this would require excessive amounts of nickel for stability and corrosion resistance, adding to the cost of the alloy but not improving its machinability, or excessive copper which would tend to make the alloy hot short if the desired stability and machinability are to be obtained; also, more than 2% manganese is required since it adds substantially to the machinability of the alloy. There cannot be more than about 10% manganese, since the amount of copper that could be used would be correspondingly reduced be cause of hot shortness problems, and also there is the problem of reduced sulfur retention. With less than about 50% copper, the machinability is greatly reduced, which would require excessive amounts of manganese to compensate for this which, in turn, reduces the amount of sulfur retained. With more than about 3% copper, the alloy tends to be hot short unless the manganese content is maintained low, and the copper has a lesser effect on the suppression of delta ferrite than manganese or nickel. When the carbon and nitrogen contents exceed about .15% and .10% respectively, they adverselv affect the machinability of the alloy as well as adversely affecting the corrosion resistance.

While an alloy falling within the broad limits described above and wherein the delta ferrite-forming potential is less than 10% and the copper does not exceed 3.85-0.18 (percent Mn S), an austenitic stainless steel of superior machining characteristics is produced. As was indicated previously, the principal element relied on for ease of machining is sulfur. As can be seen from Table I, where the sulfur is in the range of 30% to .40%, an alloy having a machinability rating based on drill life three to four times as good as AISI Type 303 is produced, and hence, for superior machinability sulfur contents of between .30% and .40% are desired. Where increased workability and corrosion resistance are desired, a sulfur content of between .10% and .3 is preferred.

It is easier to achieve an economic balance of element within a substantially narrower melting range. This narrower or preferred range is as follows: about .08% maximum carbon, from about 4% to 6% manganese where sulfur in the range of .30% to .40% is required, and 6% to 8% manganese where sulfur in the range of .10% to .30% is required, from to 7% nickel, from 14% to 18% chromium, and from 1.5% to 2.5 copper. Within this narrow range it is still necessary, through, to keep the delta ferrite-forming potential at less than 10, and the copper-manganese balance must be maintained according to the formulae given above for the alloy to be within the scope of this invention.

Although several embodiments of this invention have been shown and described, various adaptations and modifications may be made without departing from the scope of the appended claims.

I claim:

1. In the process for making a free machining, austenitic stainless steel consisting essentially of from a trace up to .15 carbon, from 2% to manganese, from 4% to 13% nickel, from 10% to 20% chromium. from .5 to 3% copper, from .10% to .40% sulfur, 2% max. silicon, and .10% max. nitrogen, the balance essentially iron and residual impurities, the improvement which comprises adding alloying additions, as described, to a steel melt and controlling the constituents so that the delta ferrite-forming characteristic is less than 10 according to the formula:

delta ferrite potential=percent Cr+1.5 (percent Si) .87[30(% C+ percent N)+ percent Ni +0.5(percent Mn) +0.3 (percent Cu) +1 and the amount of copper does not exceed 3.85-0.18 (percent Mn- S).

2. In the process for making a free machining, austenitic stainless steel consisting essentially of from a trace up to .15% carbon, from 4% to 6% manganese, from 4% to 13% nickel, from 10% to 20% chromium, from .5 to 3% copper, from .30% to .40% sulfur, 2% max. silicon, and 10% max. nitrogen, the balance essentially iron and residual impurities, the improvement which comprises adding alloying additions, as described, to a steel melt and controlling the constituents so that the delta ferritforming characteristic is less than 10 according to the formula:

delta ferrite potential=percent Cr+ 1.5 (percent Si) .87[30(% (3+ percent N)+ percent Ni +0.5 (percent Mn) +0.3(percent Cu)] +1 and the amount of copper does not exceed 3.85-0.18 (percent Mn- S).

3. In the process for making a free machining, austenitic stainless steel consisting essentially of from a trace up to .15% carbon, from 6% to 8% manganese, from 4% to 13% nickel, from 10% to 20% chromium, from .5 to 3% copper, from .10% to .40% sulfur, 2% max. silicon, and .10% max. nitrogen, the balance essentially iron and residual impurities, the improvement which comprises adding alloying additions, as described, to a steel melt and controlling the constituents so that the delta ferrite-forming characteristic is less than 10 according to the formula:

delta ferrite potential=percent Cr+1.5 (percent Si) .87[30(% C+ percent N)+ percent Ni +0.5 (percent Mn) +0.3 (percent O1) +1 and the amount of copper does not exceed 3.85-0.18 (percent Mn- S).

4. In the process for making a free machining, austenitic stainless steel consisting essentially of from a trace up to .08% carbon, from 4% to 8% manganese, from 5% to 7% nickel, from 14% to 18% chromium, from 1.5% to 2.5% copper, from .10% to .40% sulfur, 1% max. silicon and .10% max. nitrogen, the balance essentially iron and residual impurities, the improvement which comprises adding alloying additions, as described, to a steel melt and controlling the constituents so that the delta ferrite-forming characteristic is less than 10 according to the formula:

delta ferrite potential=percent Cr+1.5 (percent Si) .87[30(% C+ percent N)+ percent Ni +0.5(percent Mn) +0.3 (percent Cu)]+l and the amount of copper does not exceed 3.85-0.18 (percent Mn- S).

5. In the process for making a free machining, austenitic stainless steel consisting essentially of from a trace up to .08% carbon, from 4% to 6% manganese, from 5% to 7% nickel, from 14% to 18% chromium, from 1.5% to 2.5% copper, from 30% to .40% sulfur, 1% max. silicon and .10% max. nitrogen, the balance essential iron and residual impurities, the improvement which comprises adding alloying additions, as described, to a steel melt and controlling the constituents so that the delta ferrite-forming characteristic is less than 10 according to the formula:

delta ferrite potential=percent Cr+ 1.5 (percent Si) .87[30(% C+ percent N)+ percent Ni +0.5 (percent Mn) +0.3(percent Cu)] +1 and the amount of copper does not exceed 3.85-0.18 (percent Mn- S).

6. In the process for making a free machining, austenitic stainless steel consisting essentially of from a trace up to .08% carbon, from 6% to 8% manganese, from 5% to 7% nickel, from 14% to 18% chromium, from 1.5% to 2.5% copper, from .10% to 30% sulfur, 1% max. silicon and .10% max. nitrogen, the balance essentially iron and residual impurities, the improvement which comprises adding alloying additions, as described to a steel melt and controlling the constituents so that the delta ferrite-forming characteristic is less than 10 according to the formula:

delta ferrite potential=percent Cr+1.5 (percent S i) -.87[30(% C+ percent N)+ percent Ni +0.5(percent Mn) +0.3 (percent Cu)] 1 and the amount of copper does not exceed 3.85-0.18 (percent Mn- S).

References Cited UNITED STATES PATENTS 1,962,702 6/ 1934 Armst. 2,624,670 1/ 1953 Binder 125 2,697,035 12/1954 Clarke 75125 HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75128 

